Motor vehicles are generally driven in a 2-wheel drive mode and a 4-wheel drive mode.
First, the 2-wheel drive mode vehicle will be described. The 2-wheel drive mode vehicle is classified into a rear engine rear drive (RR) mode, a front engine front drive (FF) mode, a front engine rear drive (FR) mode, etc., in accordance with the position of the engine and whether the driving shaft is connected to the front wheels or rear wheels.
Since the RR mode is widely used in a sports car, rather than a passenger car or a SUV type vehicle, the 2-wheel drive mode vehicle may be largely classified into an FF mode vehicle and a FR mode vehicle.
Both of the FF and FR mode vehicles have the engine and the transmission installed at front sides of the vehicles while having different arrangement structures. That is to say, the FF mode vehicle is generally configured such that the engine and the transmission are laterally installed on the vehicle to transmit power to front wheels through a differential integrally formed with the transmission, and the FR mode vehicle is generally configured such that the engine and the transmission are installed on the vehicle back and forth to transmit power from the transmission to rear wheels through a propeller shaft.
Next, the 4-wheel drive mode vehicle will now be described. The 4-wheel drive mode vehicle employs a clutch, a transmission and a differential to appropriately exert a driving force of the engine on all four wheels and are classified into a part time mode for switching the drive mode from a 2-wheel drive mode to a 4-wheel drive mode by driver's manual operation and a full time mode (or a full time 4-wheel drive mode) for maintaining the drive mode all the time in a 4-wheel drive mode.
A 4-wheel drive vehicle basically travels in a 2-wheel drive mode and can be shifted manually by the driver operator from the 2-wheel drive mode to the 4-wheel drive mode when necessary. The FF mode part time 4-wheel drive vehicle is configured to supply power to front wheels in the 2-wheel drive mode and distributes the power to rear wheels in the 4-wheel drive mode. The FR mode part time 4-wheel drive vehicle is configured to supply power to rear wheels in the 2-wheel drive mode and distributes the power to front wheels in the 4-wheel drive mode.
As described above, unlike the 4-wheel driving vehicle using the engine as a power source, the 4-wheel driving electric vehicle uses motors as a power source. In the 4-wheel driving electric vehicle, a front-wheel driving motor and a rear-wheel driving motor are separately installed. The power generated from one of the front-wheel driving motor and the rear-wheel driving motor is used in the 2-wheel drive mode, and both of the power generated from the front-wheel driving motor and the power generated from the rear-wheel driving motor are used in the 4-wheel drive mode.
FIG. 1 is a cross-sectional view of a conventional rear-wheel driving device for a 4-wheel driving electric vehicle.
As shown in FIG. 1, the conventional rear-wheel driving device for a 4-wheel driving electric vehicle includes a rear-wheel driving motor 10 generating rotation power for driving the vehicle, a reducer 20 connected to a rotary shaft of the rear-wheel driving motor 10, a multiple disc clutch 30 connected to the reducer 20, a differential 40 connected to the multiple disc clutch 30, and a rear wheel driving shaft 50 connected to the differential 40 and having left and right ends to mount rear wheels (not shown) thereon.
The aforementioned conventional rear-wheel driving device for a 4-wheel driving electric vehicle operates in the following manner.
If rotation power is generated from the rear-wheel driving motor 10 in 4-wheel driving, after receiving the rotation power of the rear-wheel driving motor 10, the reducer 20 transmits the rotation power to the multiple disc clutch 30.
The multiple disc clutch 30 transmits the rotation power transmitted from the reducer 20 to a case of the differential 40, thereby rotating the rear wheel driving shaft 50 connected to the differential 40.
The rear wheels (not shown) are mounted on opposite ends of the rear wheel driving shaft 50 to perform a differential function and to transmit the driving force to the rear wheels, thereby driving the vehicle.
In 2-wheel driving, the rear-wheel driving motor 10 does not generate the rotation power, and the multiple disc clutch 30 does not transmit the rotation power to the case of the differential 40, so that the rear wheels are not drawn by the driving force of the front wheels.
However, in the conventional rear-wheel driving device for a 4-wheel driving electric vehicle, since the power is transmitted using a multiple disc clutch in 4-wheel driving, a loss in the driving force may be generated due to a slip between friction plates disposed within the multiple disc clutch.
In addition, in the conventional rear-wheel driving device for a 4-wheel driving electric vehicle, in 2-wheel driving in which a rear-wheel driving motor is not driven, a portion of the multiple disc clutch connected to the case of the differential 40 may unnecessarily rotate together with the differential when the rear wheels are drawn by the driving force of the front wheels, resulting in energy loss.